Dispersion of finely divided substances in an isoparaffin apolar dispersing agent

ABSTRACT

A DISPERSION OF FINELY DIVIDED SUBSTANCES IN AN APOLAR DISPERSING AGENT CONTAINING SURFACE-ACTIVE ION-FORMING SUBSTANCES, SAID DISPERSION BEING USEFUL IN AN ELECTROPHORETIC METHOD OF DEPOSITING FINELY DIVIDED PARTICLES.

United States Patent US. Cl. 26033.6 R 4 Claims ABSTRACT OF THEDISCLOSURE A dispersion of finely divided substances in an apolardtspersmg agent containing surface-active ion-forming substances, saiddispersion being useful in an electrophoretic method of depositingfinely divided particles.

The invention relates to a dispersion of finely divided substances in anapolar dispersing agent, to an electrophoretic method using such adispersion and to an electrophotographic method using such a dispersionas a developer.

Electrophoresis is understood to mean the transport of dispersedparticles in an electric field, which transport is brought about becausethese particles carry an electric charge. Many uses of electrophoresisin polar dispersing agents are known in technology.

Charging of the particles is based on the formation of ions byelectrolytic dissociation in polar dispersing agents having a highdielectric constant such as water or alcohols whose dipolar moleculessurround the ion. The solvation energy of this process is equal to orlarger than the lattice energy of an ionic compound and renders thedissolution of such a compound in the dispersing agent possible. Toobtain a charge the colloidal particle itself must include dissociablegroups, for example, macromolecules having built-in carboxyl or hydroxylgroups, or such particle absorbing an ion of a low molecular weightelectrolyte. The mechanism of electrophoresis in polar dispersing agentsis based on methods of depositing finely divided substances on objectswhich for this purpose are arranged as electrodes and have a conductingsurface. Known technical uses are among others coating oxide cathodesand lacquering motor bodies and other objects.

In certain cases there are great limitations in using the method for theelectrophoretic deposition of particles. Substances which are soluble inwater or react chemically in a different manner cannot be used inaqueous systems. The comparatively high natural conductivity of thesesystems bars their use in those cases wherein only a limited quantity ofthe charges generating the field is available and wherein as great apossible mass for each transported charge must be deposited.

Furthermore, it has already been proposed to use apolar organic liquidshaving a small dielectric constant and a high electric resistance asdispersing agents. The formation of ions by means of dipolar solvationin the manner described above is not possible in these dispersingagents. Small solid-substance particles may, however, be charged bybarrier layer effects, tribe-electrical effects or contact potentialdifferences, a particle having a dielectric constant which is higherthan that of the dispersing agent being positively charged and beingnegatively charged 1n the converse case. This natural charge may besuflicient for small particles (approximately 0.1 ,um) for transport inan electric field; on the other hand larger particles cannot bedeposited on a flat electrode. In this case the so calleddielectrophoresis in non-homogeneous electric fields occurs. This occursin such a manner that an uncharged or a weakly charged particle in theelectric field is polarized. In the inhomogeneous field a net attractionof gravity towards a higher field strength is obtained independent ofthe polarity of the electrode, when the dielectric constant or thepolarisability is larger than that of the surrounding dispersing agent.Th attractive force is proportional to the polarisability and to thevolume of the particle and the gradient of the square of the fieldstrength. Larger and insufiiciently charged particles can thus bedeposited on the edges of the electrodes and on irregularities and oncorners and edges of charging profiles because inhomogeneous fielddistributions occur in these areas.

For a uniform electrophoretic deposition of particles larger than 0.1 m,up to approximately 10 ,um as occur in most cases in a homogeneouselectric field between two electrodes in apolar media it must beattempted to find a possibility to sufiiciently charge these particles.The same applies to the deposition of pigments which are proportional tothe charge on differently charged dielectric surfaces as occur in thelatent charge image of an imagereproducing layer in electrostaticelectrophotography. In that case it must be possible to check andcontrol the charge of the particles satisfactorily in order to depositby means of an optimum ratio between charge and mass as many as possibleparticles per surface charge and to obtain a smooth electrophoreticalbehaviour.

German patent specification 1,047,616 describes a method of developingelectrostatic charge images wherein a dispersion of pigment particles inapolar media such as cyclohexane is used. The pigment particles areelectrically rendered selective for the image charge by adding asubstance which controls the pigment. This substance gets to surroundthe particle and to influence charging by adjusting a given ratiobetween its dielectric constant and the dielectric constant of thesolvent. Such a substance is, for example, an alkyd resin, an alkydresin modified with linseed oil or boiled linseed oil itself. The twofirstmentioned compounds must charge the pigment particle positivelywhile the last-mentioned compound must charge this particle negatively.In practice, however, it is dependent on the chemical nature of thepigment substance whether this charge is obtained. When choosing apredetermined pigment, it is very diflicult to obtain optionallypositively or negatively charged particles. These com pounds are,however, also active when fixing the pigment image during which processthey are hardened. In that case a known hardening agent, namely a soapof a heavy metal such as lead or cobalt naphthenate is used. The agentis already added to the developer solution.

In addition more has become known about the stability of suspensions inapolar media. On the one hand the steric or entropic stabilizationeffect is known and on the other hand the stabilization by electrostaticrepelling of particles charged at the same polarity. Thus, suspensionsmay be stabilized both with ionic and non-ionic substances from thegroup of surface-active substances. It was found that solutions of ionicstabilizers have an electric conductivity which is 10 times higher thanthat of the pure apolar solvents. Thus it must be possible to form ionsalso in apolar media, even at a very slight degree of dissociation. Foran ionic substance in an apolar solvent, it must be assumed that as aresult of the structure of the substances having these effects micellesare formed from a lyophilic organic acid radical and a lyophobic cation,which micelles may envelope and shield a few lyophobic cations, while afew lyophilic radicals are present in the solution. If additionalparticles are present in a suspension the lyophobic cation can beadsorbed in polar groups at the surface of the particles. The charge maythus be regarded to be a dissociative adsorption wherein a certain typeof ion is bound in an adsorption phase while the other ion is present inthe solvent.

The experiments discussed in the preceding paragraphs were performed incomparatively strong solvents such as benzene or xylene on fairly smallparticles 1 m) having polar surfaces (oxides). However, it was foundthat no satisfactory electrophoretic deposition in the sense of theinvention could be obtained under the said circumstances. The followingdifficulties occur among others: due to still unexplained conditions onthe electrodes it is difficult to obtain an adherent deposition to theelectrodes with the aid of the particles transported by electrophoreticprocess.

For electrophotographic and electrographic techniques all particles mustat an average carry the same charge of a certain polarity becauseotherwise the pigmentation proportional to the charge is not assured anddeposition occurs in unwanted areas. In addition to charged particles,insufficiently charged particles frequently occur in comparatively largeparticles so that not only electrophoresis but also dielectrophoresis isobserved. As a result strong potential differences as occur at the edgesin the image are particularly strongly pigmented.

Ions of the polarity which are adsorbed at the solid particle may alsooccur in micelles so that the charge image for the deposition cannot beutilized optimally because the micelles neutralize the charge unused.

An object of the present invention is to provide a dispersion of solidparticles in apolar solvents which does not give the above-mentioneddrawbacks and wherein solid particles of an arbitrary material naturesuch as metals, semiconductors and dielectrics having a grain size ofapproximately 0.01-10 m and in special cases even larger particles arecharged.

According to the invention the dispersion of a finely divided substancein an apolar dispersing agent having an electrical conductivity which issmaller than is characterized in that the dispersion includessurfaceactive ion-forming substances which are soluble in the dispersingagent and are dissociable in such a manner that their conductivity in a10- molar solution is greater than l' SZ- cmr and preferably between 1l0 and 1 10- .2- cm.- and that these substances comprise bivalent ormultivalent ions.

Such a multivalent compound dissociates into an ion consisting of themetal atom and the remaining lyophilic acid radical and into anoppositely charged lyophilic radical itself. A metal ion including aremaining lyophilic acid radical formed in this manner may be stabilisedeither into a micelle or into an adsorption phase built up similarly inan easier manner than a single alkali ion. Alternatively mixtures ofseveral of these substances may yield particularly favourable results.The concentration of the substance to be added must lie between certainlimits: When this concentration is too low dielectrophoresis occurs andwhen it is too high the charges of a charge pattern are compensated bymicelles of the same polarity as the pigment particles so that only asmall deposit thickness can be obtained. A series of simple testsenables anyone skilled in the art to determine the active concentrationrange for each individual case.

In a further embodiment according to the invention of a dispersion inisoparafiins this dispersion includes a substance which intensifies andstabilises the dissociative adsorption of the ionic compound due to theformation of a special adsorptive phase and which renders control ofthis phase within comparatively large ranges of concentration and chargepossible. This substance comprises a soluble non-ionic macromolecularcompound whose molecules include side chains of at least 4 carbon atoms.Particularly suitable are substances having side chains of 1020 carbonatoms. Suitable for suspension in isoparaffins are, for example,polymethacrylic acid esters having a molecular weight of 10 -10",polyacrylic acid esters, polyalkyl styrenes, polyvinyl alkyl ethers andcopolymers or polymethacrylates having cyclic amides and fumarates.

By adding such a macromolecular compound which is denoted by the termintensifying and stabilising agent it is possible to render potentiallyionic substances usable by which a usable suspension as such for anelectrophoretic deposition within the scope of the present inventioncannot be manufactured. Dispersions can be stabilised in an optimum andreproducible manner with these substances. In this manner surfacecoatings of more than 2 1ng./cm.- can be obtained by means of anelectrophotographic technique. The usual quantities ofelectrophotographically deposited pigment are slightly more than 0.1mg./cm. It is alternatively possible to charge one and the same pigmenteither unambiguously positively or unambiguously negatively by usingdifferent ionic substances.

The quantities to be used are dependent on the concentration of theparticles, the nature and the size of their surface and on the manner ofdispersing. As a result of the synergistic action of the two additions aquantity of ionic substance is used which is much smaller than when onlythe last-mentioned substance is used. The intensifying and stabilisingagent is first dispersed with the pigment, optionally together with thepigment and the added ionic substance. The optimum activity frequentlycannot be obtained if this substance is added to the suspension at alater stage.

Ionic substance which are soluble in isoparafiins are, for example:alkaline earth alkyl sulfonates containing 24-32 carbon atoms such asCa(SO C H basic alkaline earth alkyl sulfonates such as wherein R may bea mixture of different radicals, salts of fatty acids such as Mg, Ca andBa oleates, Conaphthenate, salts of alkyl salicylic acid such as calciumdiisopropyl salicylate, alkaline earth and aluminum salts of alkylesters of sulphosuccinic acid, such as Cadodecyl sulphosuccinate orcetyl-, octyl or stearyl titanate. Mixtures of these substances mayalternatively be active.

Particularly active combinations of these ionic substances are mixturesof an alkyl salicylate including cetyl titanate or an alkyl salicylateincluding an alkyl sulphosuccinate.

A dispersion has already been described hereinbefore wherein pigmentparticles are dispersed in cyclohexane while adding an alkyd resin, analkyd resin modified with linseed oil or linseed oil itself including Pbor C0 naphthenate as a hardening agent, which, however, is not based onthe recognition of the present invention. These combinations aretherefore explicitly excluded from the rights applied for.

The dispersion according to the invention is described hereinafter withreference to a few examples.

EXAMPLE 1 A mixture of solid substances having the followingcomposition:

0.5 g. of Zn-activated ZnCdS having a grain size of approximately 1.5,um

0.015 g. of a mixture of Ca alkyl salicylate and Ca dodecylsulphosuccinate 0.08 g. of a copolymer of polymethacrylate includingcyclic amides having a molecular weight of several times was dispersedin 100-300 gms. of isoparafiin (C -C having a boiling range of between180 and 210 C. After electrophoresis at a field strength of 50-100 v.cm." this dispersion yielded very homogeneous, compact layers having asmooth surface. These layers are suitable for luminescent screens havinga high resolving power.

In this method the solid substance may be replaced by any othersubstance while the nature and the concentration of the additions may bechanged and adapted to the envisaged object and to the condition of thesolid substance. Luminescent substances which are soluble in water suchas, for example, cesium iodide may be deposited advantageously in thismanner. Metals, for example, silver may be deposited for intensifyingmetallic conductive paths. Ceramic and metallic pulverulent magneticmaterials may be used for the manufacture of magnetic layers andstructures.

EXAMPLE 2 An electrophotographic developer was prepared by dispersingthe following mixture:

1 g. of gas soot which was superficially rendered oxygen-containing bypost-oxidation and had a particle size of approximately 0.1 pm in thedeveloper; primary particle size 0.03 ,um.

0.8 g. of basic Ba sulphonate of C H in 1000 gms. of isoparafiin of thequality according to Example 1. Very fine-grained images were obtainedwith this very sensitive developer. Zinc oxide papers which are usuallycharged to several hundred volts of surface potential by means of acorona discharge could be developed to full density even at 30-70 v.This technique is a condition for a high quality reproduction of halftones. Zinc oxide papers having a great charge produced stained andmottled images as a result of an imhomogeneous charge distribution andadditionally showed a step gradation.

When being used as a negative developer the exposed areas are pigmentedon the negatively charged zinc oxide; thus this developer is suitablefor a negative-positive process.

Positively charged soot developers are very difficult to manufacture.Usually soot-pigmented synthetic resins or pastes having a high contentof synthetic resin are used as basic material for such developers. Sucha developer may, however, be manufactured easily from pure soot havingfew additions with the aid of the stabilising agents. This developerhas, for example, the following composition:

1 g. flame soot 0.1 g. Ca-diisopropyl salicylate 0.4 g. polymethacrylateacid alkyl ester having side chains of 10-20 C-atoms and a molecularweight of approximately 7 /2 x10 dispersed in 1000 gms. of isoparafiinof the quality according to Example 1.

In many technological processes photographic methods are used to obtainpatterns on carrier materials by means of additive or subtractivemethods (ph'otoetching, photohardening method). It is evident to obtainthe pattern in the form of a charge image for additive methods whichimage may be obtained electrophotographically or in some cases also by adirect supply of charge. To coat this charge image with a layer ofsufiicient thickness of 6 the desired material with the aid of theabove-described method, strict requirements are imposed on the optimumcontrollable charging capacity of the particles.

An interesting use of the dispersion according to the invention consistsin the manufacture of the luminescent layer on the inner side oftelevision picture tubes and particularly of a pattern comprising threeluminescent materials for the screens of colour television picturetubes. For this purpose the inner side of the picture screen is providedwith a conducting layer and a photoconducting organic layer is providedon said layer; an electric charge is supplied by a corona discharge tothe photoconducting layer Whereafter the charged layer is exposed inaccordance with the desired pattern and is treated with the liquiddispersion of luminescent materials according to the invention. Thisprocess is repeated for the two other luminescent materials. Theconducting layer and the photoconducting layer are finally removed byheating while simultaneously the luminescent layers are compacted bysintering.

The following dispersions are described as examples for the manufactureof screens for colour television picture tubes according to theabove-mentioned methods.

A mixture having the following composition:

3 g. activated ZnS (blue luminescent material) grain size approximately5 pm,

0.03 g. of a mixture of Ca-diisopropyl salicylate and cetyl titanate(1:1)

0.5 g. of a polyacrylic acid alkyl ester, molecular weight several times10 was first dispersed in:

g. isoparaffin (C -C boiling range 40-60 C.),

while using ultrasonic treatment, whereafter the dispersion obtained wasdispersed in 250 gms. of isoparafiin (C C boiling range -210 C.) and wasdiluted to 1500 mls. for use. The dispersing was carried out by anultrasonic treatment so as to expose the luminescent material as littleas possible to mechanical load which reduces its light output. Toinfluence the cavitation for obtaining an effective irradiation in afavourable sense, a low boiling isoparaffin was used first.

A mixture of the following composition:

3 g. activated YVO (red luminescent material) grain size approximately 5,um,

0.003 g. of a mixture of Ca-all ylsalicylate and Cadodecylsulphonate.

0.2 g. of a polymethacrylic acid alkyl ester having side chains of C Hand a molecular weight of 8x10 was dispersed in isoparaffin likewise asthe blue luminescent material. In this dispersion the particles werepositively charged.

The following mixture was used for the dispersion of a negativelycharged (green) luminescent material.

3 g. activated ZnCdS, grain size approximately 5 ,um,

0.2 g. Ba-oleate 0.3 g. of a polyalkyl styrene having alkyl side chainsof more than 4 carbon atoms.

What is claimed is:

1. A dispersion in at least one isoparafiin apolar dispersing agenthaving an electrical conductivity of less than l0 *S2- cm.- of finelydivided particles capable of being deposited on an electrode byelectrophoresis and having a particle size of about 0.01-10 m, at leastone surfaceactive ion-forming substance, said surface-active ion-formingsubstance being soluble in said dispersing agent, having a dissociationcapacity such that its electrical conductivity in a 10- molecularsolution lies between about 1 10- and 1 l0 0- cm.- and comprisingbivalent and multivalent ions and a soluble nonionic macro-molecularcompound having at least one side chain of at least 4 carbon atoms.

2. The dispersion of claim 1 wherein the dispersing agent is a mixtureof isoparaffins and the soluble nonionic macro-molecular compound hasside chains of 10-20 carbon atoms.

3. A dispersion of claim 2 containing a mixture of an OTHER REFERENCESalkyl salicylate and cetyl titanate as surface-active ion schwanz et atsurface Active Agents, VOL I, Intep formmg P P science Publishers, Inc.,N.Y., 1949, pp. 30-33 and 527 4. A dispersion of claim 2 contammg amixture of an 7 alkyl s a1icy1ate and an alkyl sulphosuccinate assurface- 5 Schwartz et all: surface Active Agents and Damp actlve 10Hformmg substancesgents, v01. 11, Interscience Publishers, Inc., N.Y.,1958,

. 715. References Cited p UNITED STATES PATENTS 10 ALLAN LIEBERMAN,Primary Examiner 2,898,279 8/1959 Metcalfe et a1. 204181 US Cl 2,907,67410/1959 Metcalfe et a1. 117-93 3,450,655 6/1969 Spiller 204-18111793;24181;25238,309

